Thermal spraying in general, and plasma spraying in particular, is a powerful technique widely used to produce protective coatings on a large variety of substrates. For example, thermal barrier coatings are plasma sprayed in producing aircraft engines, and ceramic and metal coatings are thermally sprayed for various purposes. Coating properties depend upon many spraying parameters, some of them being related to the spray gun operation. Consequently, spraying process control has been implemented by monitoring and regulating gun input variables as arc current and power, arc gas flow rates, powder feed rate and powder gas pressure, to keep them at predetermined optimum values. This control approach is quite complex because a large number of interrelated input variables must be monitored, while some variables, such as electrode wear, cannot be monitored at all.
On-line measurement of the variables which directly influence the structure of thermally sprayed coatings, can provide an efficient feedback for the setting of the spray gun parameters and a diagnostic tool to detect any problems during the coating operation.
It is well known to those skilled in the art that thermal spraying denotes a number of techniques besides plasma spraying, e.g. arc spraying, HVOF and flame spraying.
Collecting information about hot particle flow may also be useful in other applications, e.g. in the production of metallic powders by gas atomization.
Two types of techniques are available to perform an in-flight particle velocity measurement. In the first type, the velocity information is obtained from light impinging upon and reflected by the particles and then detected by an appropriate sensor. Laser based techniques, such as laser Doppler anemometry and laser dual focus velocimetry, are included in this first type of techniques. They use intense laser light beams to form interference fringes, or use two focused light beams in the measurement region. When the particle trajectory intercepts the measurement region, the reflected light intensity is modulated as the particle travels through the intensely illuminated zones and the velocity is computed from the modulation period. Periodic light distributions may also be obtained using a high intensity incandescent source and a Ronchi grating. This technique is not attractive as being bulky and requiring high intensity light sources.
In patents DE 2401322 and EP 150658, a source of light is guided by a bundle of optical fibers to the jet stream from which the reflected light is transmitted back by two other optical fibers and captured by two detectors. The velocity is obtained from the fiber spacing and by calculating the time delay by cross-correlation. Since the system uses reflected light, it is limited to a very small measuring volume and the sensor must be relatively close to the jet which may cause heating problems in thermal spray processes. Secondly, no information can be obtained concerning the temperature of the jet.
The second type of techniques used to perform the velocity measurement takes advantage of the thermal radiation emitted by the particles heated to a high temperature by the plasma or other heat source such as HVOF. The radiation emitted by individual particles is detected when the particles pass through the detector field of view of known dimensions. The transit time is evaluated and the velocity is computed knowing the travel length. Since the dimensions of the field of view change with the distance from the optical detection assembly, it is necessary to analyze only particles near the assembly focal plane. To do that, a laser beam or a second detection assembly focused in the appropriate region from a different angle must be used in conjunction with a coincidence detection analysis system. Such a system is complex and difficult to keep well aligned under practical operating conditions. In this same type of techniques, velocity measurements can also be performed using high speed cameras. In this case, light emitted by the particles is used to image them on a high speed film and, from these images the particle velocity is determined. Such a system can be used for laboratory investigation, but it is not suitable for real time operation in the harsh thermal-spray environment.
A somewhat similar velocity measurement apparatus using two wide angle radiation detectors is described in PCT patent application WO 834437. A timing electronic circuit is used to determine the time delay between the detectors. However, the signal processing scheme is restricted to slowly moving material because of its start-stop configuration. No temperature information can be obtained from that apparatus.
A thermal radiation signal from hot gas jet was used to measure velocity by cross-correlation technique (See G. J. Liewellyn, Acta Imeko London (1976), 351-357 and P. J. Webb, Acta Imeko London (1976), 327-336). Again no information was extracted concerning the temperature of the gas jet, even though temperature measurement was performed with one wavelength pyrometry technique using a different apparatus in the publication by Webb.
U.S. Pat. No. 5,180,921 issued to Moreau et al. describes a control approach in which the temperature and velocity of the sprayed particles are monitored before their impingement on the substrate. The system of Moreau et al. has a sensor head attached to the spray gun, an optical fibre transmitting the collected radiation to a detection apparatus which incorporates two photodetectors. A two-slit mask is located in the sensor head at the end of the optical fibre. For temperature measurements, the radiation emitted by the particles and collected by the sensor head is transmitted to the photodetectors, filtered by interference filters at two adjacent wavelengths. The particle temperature may be computed from the ratio of the detector outputs. To measure the velocity, the two-slit system collects radiation emitted by the in-flight particles tracelling in the sensor field-of-view, which generates a double-peak light pulse transmitted through the optical fibre. The time delay between these two peaks may be evaluated automatically and the particle velocity computed knowing the distance between the two slits.
In conclusion, except for the Moreau U.S. Pat. No. 5,180,921 in certain conditions, none of the above mentioned systems are capable of or adapted for simultaneous measurements of both velocity and temperature in typical industrial thermal spray environment.
While the system of the Moreau et al. patent is useful, the system is designed to measure accurately the temperature and velocity of individual particles. Consequently, the measuring volume is small and the number of particles in that volume per unit time is also limited. Due to the small measuring volume, in order to obtain an average picture, the jet must be divided into many smaller regions. This can make the analysis unduly lengthy. The fact that the number of particles in the measuring volume is limited, imposes a maximum powder feed rate, on the order of 5 kg per hour, which can still be analyzed accurately.
U.S. Pat. No. 5,317,165 to Montagna describes an apparatus which uses two fiber bundles to determine electromagnetic radiation emitted by the flame of a burner. The apparatus serves to detect the presence and quality of the flame.
U.S. Pat. No. 5,654,797 to Moreau et al. is concerned with the evaluation of the diameter of thermally sprayed particles.
Other systems and methods for detecting, measuring or monitoring temperature or velocity of in-flight particles are described in PCT application No. WO 834,437; DE 2,401,322; EP 150,658; Swancke et al., Proceedings of the 8th National Spray Conference 111-116 (Sep. 1995); Mishin et al., J. Phys. E. Sci. Instrum., 20 (1987) 620-5; and U.S. Pat. No. 4,656,331 to Lillquist. The various known techniques to perform the measurements of the properties of in-flight particles are discussed in the above-cited U.S. Pat. No. 5,180,921 the specification of which is being incorporated herewith by reference.
It is an object of the present invention to provide a method and apparatus for monitoring certain characteristics, and particularly temperature and velocity of thermally sprayed particles in a plasma jet during flight between a thermal jet source, e.g. a plasma gun, and a substrate.
It is another object of the invention to provide a method and apparatus as defined above, enabling the extension of prior art methods to higher powder feed rates and to lower temperatures, while being relatively simple and easy to operate.